(1) Field of the Invention
The present invention relates to a method of producing single crystal, and to single-crystal diamond obtained by the method.
(2) Description of the Related Art
Diamond possesses outstanding properties as semiconductor, and is hence expected to be used as a material for semiconductor devices such as high-frequency high-output devices, ultraviolet radiation luminescence devices, and the like. To bring such semiconductor devices into practical use, it is essential to supply large single-crystal substrates suited to device fabrication processes at low cost.
The growth of single-crystal diamond is mainly performed by high-pressure high-temperature synthetic methods, gaseous phase synthetic methods, and the like. Of these methods, high-pressure high-temperature synthetic methods wherein diamond is deposited and grown from a high-pressure high-temperature solvent have enabled the practical growth of carat (gemstone)-class large single crystals. However, these methods only produce single-crystal diamond substrates of about up to 10 mm diameter due to the limitations of the apparatus size, and it is therefore difficult to produce larger single-crystal diamonds.
In contrast, microwave plasma CVD is known as a promising gaseous phase synthetic method. In this method, the growth of single-crystal diamond takes place under a reduced-pressure atmosphere in a stream of hydrogen and methane gases, using plasma formed by microwave electric discharge. However, this method poses problems in that the growth face barely enlarges during the crystal growth process as occurs in other semiconductor materials, and that the growth rate is extremely low: only up to about 10 μm per hour. For these reasons, it requires very long processing times when attempting to produce large single-crystal diamonds, and is therefore unfeasible for producing large single-crystals.
As reported recently, in homoepitaxial growth of diamond by microwave plasma CVD, the growth of single-crystal diamond at a growth rate exceeding 100 μm per hour is possible by adding a small amount of nitrogen to a reaction gas of hydrogen and methane (Chayahara, Y. Mokuno, Y. Takasu, H. Yoshikawa, N. Fujimori; Diamond Relat. Mater. 13 (2004), 1954-1958).
However, preferentially {100} surfaces grow in this method, and it is difficult to enlarge crystal size by expanding the grown surface. Further, polycrystallization take places at the edges during the crystal growth process, thereby actually reducing the grown surface and failing to grow long crystal.
Furthermore, abnormal grain grows during the process of crystal growth from which hole-like (pipe-shaped) defects are formed and propagate in the growth direction. Once such defects are formed, the defects cannot be filled in by subsequent crystal growth.
For these reasons, it is difficult to grow a large single-crystal diamond by homoepitaxial growth using a microwave plasma CVD method.
By the way, when diamond crystals are grown at high growth rates by a microwave plasma CVD method, polycrystalline diamonds may deposit on the substrate holder. The polycrystalline diamonds tend to separate and be heated or shattered during the crystal growth. As a result, problems arise in that measuring the growth temperature using a radiation thermometer becomes difficult, polycrystalline diamond grains are taken into crystals by which defects are formed, etc. In view of this, the growth of film having a certain degree of thickness requires steps of growing a crystal to a predetermined thickness, discontinuing the growth, cleaning the substrate holder, followed by regrowing the crystal and repeating these steps to enlarge the crystal.
In this repetition of growth, although it is most desirable to polish the grown surface of the diamond evenly at every growth, such processing is difficult because a diamond is the hardest material. Therefore, attaining a polished surface having the surface smoothness suitable for regrowth is very time and cost consuming.